1. Field of the Invention
This invention relates generally to current sensing circuits, and more particularly, to a current sensing circuit having high power vertical MOS transistors.
2. Background Art
It is often desirable to sense a large current flowing in a load in order to either control that current or to prevent it from exceeding certain bounds. For example, the output current from a power supply may be limited to prevent damage to the power control devices in that supply. It may also be desirable to control the current through a device rather than the voltage applied to it, in a constant current configuration.
One previously known technique is to insert a sense resistor into the high current path and sense the voltage generated across such a device. The voltage drop across the resistor is proportional to the current flowing through it. The disadvantages of such a method are high power dissipation and excess voltage drop across the sense resistor.
A previously known circuit, U.S. Pat. No. 4,319,181, senses the current by other means. The circuit senses current by means of sampling the current through a portion of the power device. Since the power device is monolithic, the various sections of it carry proportionate current. The sensing of the current sample is accomplished by a first bipolar transistor coupled in series with a sensing resistor between an input and output terminal. A second bipolar transistor is coupled between the input terminal and output terminal, wherein a larger portion of the load current is carried through the second transistor. Lateral collectors of the transistors are used to sense a portion of the current carried by substrate collectors which provide the output. Lateral collectors are connected to a current mirror which provides an output indication of when the output current exceeds a predetermined amount.
MOS devies can not use lateral drains in the same fashion that bipolar devices can use lateral collectors. As a result, this method can not be employed by a MOS device to sense current. In a vertical power MOS device, it is generally difficult to obtain a portion of the drain current because it is flowing vertically into the device substrate. Thus, a need exists for these devices which are generally useful for vertical MOS devices but not limited to that vertical structure.